Remove CommandDefinition class

In preparation for traits support, remove CommandDefinition class
and incorporate the missing functionality into CommandDictionary.

BUG: 25841719
Change-Id: Iead48aa0503e9de6061c4c1588b0b930dd82c8d0
Reviewed-on: https://weave-review.googlesource.com/1680
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
18 files changed